The invention relates to a method of producing a multilayer microelectronic substrate.
Magnetodielectric materials, for example with nickel oxide NiO and ferrous oxide Fe2O3 as the main components of a ferrite, form an important basic material for components used in electronics and telecommunications. They are used in components which are described as inductors and transformers in the widest sense. They are produced in that a pre-sintered mixture of powdery raw materials is pressed into the desired shape, after which conversion into a ceramic component takes place by firing.
Ferrites also have favorable properties in the high-frequency or microwave ranges, making them well suited for the construction of components of this kind. The electromagnetic radiation can penetrate into the ferrites to a considerable extent, and is thereby modified in a predetermined manner through interaction between the magnetic and electrical field components of the wave and the dielectric and magnetic properties of the material. High-frequency filters. e.g. gyrators, insulators or components for the absorption of electromagnetic interference, can be constructed thereby, these being used in telecommunications devices, for example in power amplifiers for high-frequency modules.
An important technology for the production of microelectronic substrates with a high allocation density is the so-called Low Temperature Cofire Ceramics technology, hereinafter referred to as LTCC technology. Insulating ceramic films, which normally also contain openings for the vertical connections, are hereby printed with metallic conductor patterns, whereupon the laminated films are fired at temperatures between 850° C and 900° C.
Ferrite materials hitherto available have sintering temperatures that are significantly above 1200° C. These high sintering temperatures prevent ferrite materials from being used in LTCC modules, since they cannot be sintered jointly with the ceramic films used in this case.
A further problem of LTCC technology is the occurrence of shrinkage during sintering as a result of the reduced porosity of the ceramic material. The shrinkage hereby is approximately 15 to 18% in all directions. Of even greater importance is the fact that, in mass production, a further 15% relative tolerance exists in respect of the shrinkage obtained. This means, for example, that the location of an external contact point on the surface of the fired substrate can be predicted only with a probability area having a radius which increases as a measure of the shrinkage or shrinkage tolerance. This makes the automatic arrangement of chips or other discrete components of the circuit on the surface of the LTCC substrate very difficult. Large areas for the external contact points in order to circumvent this problem preclude further miniaturization. In addition, the affixing of discrete components of the latest generation 0201 standard is also problematic, if not completely impossible.
Solutions are known in the prior art that are intended to counteract the problem of shrinkage. To this end, as described in, for example, TW-A-398060, a further flexible layer, which is intended to prevent uneven shrinkage and which consists of, for example, a ceramic sinterable at high temperature, is applied to the actual LTCC laminate. This layer is removed again on conclusion of the sintering process. Shrinkages are thereby achieved which lie between only 0 and 1%, and are thereby considerably below the values specified above. The shrinkage tolerances are also said to be reduced hereby. Shrinkage will then occur only in the z-direction perpendicular to the x-y plane of the laminated films.
The use of a multi-stage sintering process for producing a ceramic body is known from WO 00/04577. At least two materials, which differ from one another at least in respect of the temperature interval necessary for compression, are used hereby. There is no temperature range in which the first ceramic material and the second ceramic material compress simultaneously. The shrinkage of the first ceramic material is already concluded before the shrinkage of the second ceramic material begins. With a method of this kind, a microelectronic substrate in which a dielectric material with a composition of the generic type specified above is embedded between two lower compression films, can be produced with improved shrinkage behavior, whereby the construction of passive components in the LTCC structure is possible.
The modification of ferrites by means of auxiliary agents in order to improve their electrical and magnetic properties is known.
For example, DE 199 24 354 A1 describes a magnetodielectric material which contains as its main components manganous oxide, zinc oxide and ferrous oxide, to which a eutectic mixture of lead (II) oxide and tungsten (IV) oxide is added in a quantity between 0 and 1500 ppm. It is mentioned that this eutectic mixture maintains the sintering temperatures at a lower level, so that smaller grain sizes are achieved, which in turn give rise to lower magnetic losses. The sintering temperatures referred to in DE 199 24 354 A1 are all above 1200° C.
SU 661 620 A describes a nickel-ferrite material which, in addition to stannic oxide and cobalt oxide, also contains tungsten oxide in a proportion of 0.01 to 1 mol %. In order to improve the initial magnetic permeability and the coefficient of frequency tuning, it also contains 0.2 to 2.0% by weight lead oxide.
It is known from SU 378 969 A that the addition of zinc oxide reduces losses in high-frequency fields. JP 06 267 742 A discloses a nickel-ferrite material for a magnetic core which is to be used for switching high-frequency power. The added tungsten oxide is thereby credited with reducing the losses in the high-frequency band. Also concerned with the reduction of power losses is JP 05 041 315 A, which proposes to this end that a manganese-zinc-based ferrite mixture containing silicon dioxide should be pre-treated to form a eutectic compound.
JP 09 306 716 A1 discloses the addition to a nickel ferrite of 0.2 to 0.5% by weight lead oxide or 0.05 to 0.13% by weight HBO3 or 0.2 to 0.5% by weight bismuth oxide and 0.4 to 1.0% by weight tungsten oxide, 0.25 to 0.65% by weight stannic oxide, 0.15 to 0.45% by weight aluminum oxide, 0.25 to 0.65% by weight chromium oxide and/or 0.1 to 0.25% by weight SiO2.
In the case of all these compositions, no mention is made of a consistent reduction in sintering temperature.
It is an object of the invention to indicate a method of producing a multilayer microelectronic substrate, with which method passive magnetic components can also be incorporated into the substrate using LTCC technology.
This object is achieved by a method as claimed in claim 1. Advantageous embodiments are the subject matter of the dependent claims. A microelectronic substrate is defined in claim 8 and the dependent claims related to it. The preferred use is specified in claim 15.
It is provided according to the invention that a magnetodielectric, nickel-ferrite-based material, containing bismuth oxide Bi2O3 or a eutectic mixture of lead oxide PbO and at least one further metallic oxide or of bismuth oxide Bi2O3 and at least one further metallic oxide, is used in a method as described in WO 00/04577.
Preference is given to the further metallic oxide molybdenic oxide MoO3, zinc oxide ZnO, silicon dioxide SiO2, boric oxide B2O3 or tungsten oxide WO3.
It has transpired that the bismuth oxide or the eutectic mixture should be contained in a quantity of 1 to 10% by weight of the ferrite, preferably in a quantity of more than 2 to 10% by weight and, further preferred, in a quantity of 4 to 10% by weight. The sintering temperature drops thereby if the proportion of the eutectic mixture in the ferrite is increased, so it is particularly preferred if the bismuth oxide or the eutectic mixture is added in a quantity of 8 to 10% by weight of the ferrite. In a particularly preferred composition, the ferrite contains NivZnwCoyFezO4, where 0.88≦v≦1, 0≦w≦0.8, 0≦y≦0.03 and 1.7≦z≦2.2.
Using the method according to the invention, a multilayer microelectronic substrate, which also contains passive magnetic components, can be produced, where the advantage of the low sintering temperature can be exploited. Using this method, further dielectric ceramic compositions can be used, such as substituted barium-neodymium-titanium-perovskite.